This invention relates to a control system adapted to control the operation of an ac/dc converter, particularly in applications where a pair of ac systems are linked by two ac/dc converters connected by a dc link.
Systems are well known where a pair of ac systems are linked by two ac/dc converters connected by a dc link. For instance propulsion systems for ships can include such a link wherein an ac supply of fixed frequency is fed into an ac/dc converter acting as a rectifier to produce dc. This dc is then fed into an ac/dc converter acting as an inverter to drive a motor. The advantage of this arrangement is that the firing angle of the inverter can be controlled to adjust the speed of the motor without affecting the first ac system.
Dc links are also known to exist between the supply grids of neighboring countries.
However, as is well known the ac/dc converters can produce electrical noise at harmonic frequencies of the ac supply frequencies. These harmonic frequencies can pass through the ac/dc converters and give rise to cross modulation frequencies on the ac systems. These cross modulation frequencies appear as noise on the ac systems, that is they can lead to voltage flicker (causing lamps, etc. to flicker) and can lead to resonances in electro-mechanical systems. An example of such an electro-mechanical system is a turbine/generator arrangement provided in power stations. The turbine and generator are linked by a shaft which has a natural frequency. The natural frequency of this system is of the same order as the cross modulation frequencies and it is theoretically possible that the system (in particular the linking shaft) could be damaged through resonance of the system due to excitation by the cross modulation. It is therefore clearly advantageous to remove this cross modulation.
According to the invention there is provided an auxiliary control system adapted to control at least one of the converters of an ac/dc system comprising a pair of ac/dc converters with a dc link extending between them linking two ac systems, at least one of said converters comprising a plurality of switches, a main control means providing converter operating signals to said switches, said auxiliary control system being arranged to take an electrical power or reactive power measurement of one of the ac systems and in accordance with that measurement cause the controlled converter to operate so as substantially to remove from the ac system a cross modulation noise component caused by interaction of electrical noise from the two converters.
Conveniently the auxiliary control system is additional to the main control systems of the converters.
An advantage of such an auxiliary control system is that the cross modulation component that had previously caused interference within the ac electrical systems connected by the dc link can be removed. This can help to remove or reduce the likelihood of damage to electro-mechanical systems within those ac systems or unwanted effects such as light flicker.
The controlled converter may be provided with main control means providing converter operating signals or pulses.
Preferably the auxiliary control system comprises demodulating means, integrating means and modulating means, and can include an input which is a function of said power or reactive power measurement and an output to modulate said control means of said main controlled converter.
In one embodiment the auxiliary control system comprises an integral two-axis ac servo system comprising aforesaid demodulating, integrating, and modulating means and said input being input to said integral two-axis ac servo system arranged to provide said output.
At least one or each of said ac/dc converters may be a thyristor converter or a voltage-source converter.
In one embodiment gain and phase settings of said auxiliary control system may be substantially fixed.
In another embodiment gain and phase settings of said auxiliary control system can be varied in dependence on at least one measured value of an electrical phenomenon in said ac/dc converter according to a pre-determined characteristic. Said phenomenon may be the dc voltage of said converter and/or said phenomenon may be the dc current.
Gain and phase settings of said control system can alternatively be set to values substantially equal to the reciprocal of a measured complex value of total loop gain of said control system and ac/dc converters and ac systems excluding said integrating means.
An automatic measurement of said total loop gain may be carried out after any substantial disturbance in the operation of said ac/dc converters and in which said measurement remains effective in determining said gain and phase settings until the next such disturbance.
Said automatic measurement of total loop gain may be by injecting pre-determined signals into said modulating means and measuring resulting outputs of said de-modulating means.
Advantageously two said total loop gain measurements may be performed in succession with different injection signals and a result of said gain measurements is calculated from a difference between the said two gain measurements.